Automotive vehicle body manufacturing operations include the joining together of two sheet metal layers by resistance spot welding. The spot welding operation is typically accomplished by assembling the sheet metal panels in a suitable fixture or other clamping apparatus and then pressing welding electrodes against opposite sides of adjoining metal panels. The weld electrodes provide both clamping force and also communicate weld current to the point of contact with the metal panels.
The weld gun is typically operated by a robot so that a succession of welds can be made in a particular manufacturing cell. Manufacturing efficiencies demand that the welds be made in very rapid succession as fast as possible, to thereby reduce the number of welders and robots that are required to assemble the vehicle body.
Modern motor vehicles may typically employ both steel panels and aluminum panels in order to optimize the strength and weight characteristics of the vehicle body. Thus the welding operations for a vehicle body may include both the welding of aluminum panels to aluminum panels and also the welding of steel panels to steel panels. Steel panels are often galvanized with a zinc coating as a corrosion preventative.
With regard to the spot welding of steel, it is known that over a number of welds a copper electrode will soften and mushroom in shape because of the combined action of clamping pressure, high temperature and the alloying of the copper with the galvanized steel surface. Accordingly, the prior art has recognized that when the copper electrode is used to weld steel or galvanized steel, the electrode must be periodically dressed in order to reshape the electrode to extend its useful life and maintain weld quality.
With regard to the use of a copper electrode to weld aluminum panels, it is known that a buildup of aluminum from the aluminum sheet onto the electrode face can result in the formation of a low-melting-point Cu—Al eutectic that eventually produces pits in the electrode's surface. To minimize the pitting reaction, the prior art has recognized that the copper electrode used for welding aluminum panels needs to be occasionally resurfaced in order to remove the contamination from the surface thereof.
Thus, with respect to welding aluminum to aluminum, U.S. Pat. No. 6,861,609 assigned to the assignee of this invention provides for the texturing of the electrode face by blasting with small grit particles or sanding with course abrasive paper in order to clean and restore the face of the copper electrode. In addition, U.S. patent application Ser. No. 11/536,001, filed Sep. 28, 2006, also assigned to the assignee of this invention discloses a method for using a tool to dress the tip of the copper electrode and make a series of concentric rings thereon. Cutting or dressing the surface during the formation of the concentric rings was designed to both clean the electrode surface and produce a texture.
Thus, although the prior art has developed techniques for welding steel to steel, and techniques for welding aluminum to aluminum, the welding of steel to steel and aluminum to aluminum using one welding gun has not been done for two primary reasons. First, welding electrodes designed to spot weld one of the materials are typically not suitable for spot welding the other material. For example, a ballnose electrode used to spot weld steel causes excessive sheet deformation and weld metal expulsion when used to spot weld aluminum. Second, contamination of aluminum panels by iron-containing particles transferred by the welding electrode could cause galvanic corrosion and premature panel deterioration.
It would therefore be desirable to provide further improvements in the welding process so that a single weld gun, using the same pair of copper electrodes could make both welds of steel to steel panels and also aluminum to aluminum panels without causing the problems given above.